The pilots of civil and military airplanes have a screen available on board in the cockpit, which screen is connected to an on-board radar for displaying a representation of the weather situation in front of the airplane. The radar operates by transmitting a radiowave signal in front of the airplane. When that signal meets an obstacle such as droplets of water in suspension in the atmosphere, it is reflected back to the airplane and received by the radar. By measuring the time between the transmission of the signal and the reception of the echo, it is possible to determine the time taken by the echo to reach the antenna of the airplane and thus the distance to the obstacle forming the source of the echo. Furthermore, by measuring the intensity of the reflected signal, the radar determines the density and the size of the droplets.
A difficulty arises when another aircraft lies in the field of coverage of the radar. That other aircraft, referred to herein as an “intruder” (without any associated notion of hostility) also constitutes an obstacle that reflects the radar signal back to the airplane that transmitted it. Such a radar echo is then detected by the on-board weather radar and it is displayed on the screen as though it were a cloud. That phenomenon occurs even when the intruder is at a range of several tens of nautical miles from the radar. Furthermore, that phenomenon is particularly visible on the screen when flying in good weather. It gives rise to a spot on the screen that looks like a small cloud at the location of the intruder, even though there is not a cloud in the sky. That defect requires the crew of the airplane either to interpret the anomaly as being an anomaly by performing verifications, or else to believe that the representation the radar screen does indeed correspond to a cloud and thus to be wrongly informed about the nature of the obstacle and the danger that it might represent.